/Engine/Source/Runtime/Engine/Classes/Engine/Texture.h
Get the dimensions of the largest mip of the texture when built for the target platform accounting for LODBias and other constraints
void GetBuiltTextureSize ( const [ITargetPlatform](API\Developer\TargetPlatform\Interfaces\ITargetPlatform) * TargetPlatform, int32 & OutSizeX, int32 & OutSizeY ) const